Nanowires Are Especially Attractive Biology Essay

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In the last ten years the nanomaterials science and technology have represented one of the most attractive interdisciplinary science researches. The growing interest for the nanoscience domain resides in potential applications in physics, chemistry, biology and electronics. Nowadays, the research in the nanomaterials field takes advantage from important funding since they are the basis for the development of new technologies, devices and systems.

Nanowires are especially attractive for nanoscience studies as well as for nanotechnology applications. Nanowires, compared to other low dimensional systems, have two quantum confined directions, while still leaving one unconfined direction for electrical conduction. This allows nanowires to be used in applications where electrical conduction, rather than tunneling transport, is required. Because of their unique density of electronic states, nanowires in the limit of small diameters are expected to exhibit significantly different optical, electrical and magnetic properties from their bulk 3D crystalline counterparts. The increased surface area, very high density of electronic states and joint density of states near the energies of their van Hove singularities, enhanced exciton binding energy, diameter-dependent band gap, and increased surface scattering for electrons and phonons are just some of the ways in which nanowires differ from their corresponding bulk materials. Yet the sizes of nanowires are typically large enough (> 1nm in the quantum confined direction) to have local crystal structures closely related to their parent materials, thereby allowing theoretical predictions about their properties to be made on the basis of an extensive literature relevant to their bulk properties. Not only do nanowires exhibit many properties that are similar to, and others that are distinctly different from, those of their bulk counterparts, nanowires have the advantage from an applications standpoint that some of the materials parameters that are critical for certain properties can be independently controlled in nanowires, but not in their bulk counterparts, such as, for example, their thermal conductivity. Also certain properties can be enhanced non-linearly in small diameter nanowires, by exploiting the singular aspects of the 1D electronic density of states. Furthermore, nanowires have been shown to provide a promising framework for applying the "bottom-up" approach [1] for the design of nanostructures for nanoscience investigations and for potential nanotechnology applications.

[1]. Feynman, R. P. (1959). There's plenty of room at the bottom. From a talk reported at the web site http://www.zyvex.com/nanotech/feynman.html.

Synthesis

Bibliographical data present many synthesis methods of simple and multilayered nanowires such as: photochemical synthesis [1], catalytical synthesis [2], vapour-liquid-solid growing [3], electrochemical deposition [4-6]. The preparation of nanowires by electrochemical deposition in nanosised pores is more frequently used because of the low cost and the better energetic efficiency of process. The electro deposition is a preparation method which allows the controlled deposition from solution of metallic materials. Generally, such a solution contains dissolved salts of metals which are going to be deposited. Passing of a current through the electrochemical cell (formed by three electrodes: the reference electrode, the counter electrode and the working electrode) allows the ions migration from the electrochemical bath to working electrode and their deposition in metallic state.

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[2]. Huang, Y.; Duan, X.F.; Cui, Y. & Lieber, C.M. (2002). Gallium Nitride Nanowire Nanodevices, Nano Lettres, Vol. 2, No.2, pp. 101-104, ISSN 1530-6984.

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[6]. Xu, J. & Wang, K. (2008). Pulsed electrodeposition of monocrystalline Ni nanowire array and its magnetic properties, Applied Surface Science, Vol.254, pp. 6623-6627, ISSN 0169-4332.

Template Synthesis

The template-assisted synthesis of nanowires is a conceptually simple and intuitive way to fabricate nanostructures (Ozin, 1992; Tonucci et al., 1992; Ying, 1999). These templates contain very small cylindrical pores or voids within the host material, and the empty spaces are filled with the chosen material, which adopts the pore morphology, to form nanowires. In template-assisted synthesis of nanostructures, the chemical stability and mechanical properties of the template, as well as the diameter, uniformity and density of the pores are important characteristics to consider. Templates frequently used for nanowire synthesis include anodic alumina (Al2O3), nano-channel glass, ion track-etched polymers and mica films.

Track-Etched Membrane

The polycarbonate membranes are obtained by the "track-etch" method. This method uses the bombardment with heavy atoms of a nonporous material to create holes. This step is followed by chemical treatment to transform the holes in nanopores. The nanoporouse membrane contains cylindrical pores of uniform diameters but which are randomly distributed on its surface. The porous template that is commonly used for nanowire synthesis is the template type fabricated by chemically etching particle tracks originating from ion bombardment [1], such as track-etched polycarbonate membranes [2-5] and also mica Films [6].

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[6]. Sun, L., Searson, P., and Chien, C. L. (1999). Electrochemical deposition of nickel nanowire arrays in single-crystal mica films. Appl. Phys. Lett., 74:2803-2805.

Anodic Alumina Template

Alumina nonporous plane membranes (AAO) are obtained by anodization of aluminum foils in acids electrolytes containing bivalent or trivalent anions such as: oxalic acid (COOH)2 [1]., sulphuric acid H2SO4 [2], or phosphoric acid H3PO4 [3]. One of the methods proposed in literature, which leads to the preparation of plane and good quality membrane, is the anodization in two steps. This method was proposed for the first time by Masuda and Fukuda [4].

Porous anodic alumina templates are produced by anodizing pure Al films in various acids [5-7]. Under carefully chosen anodization conditions, the resulting oxide film possesses a regular hexagonal array of parallel and nearly cylindrical channels. The self-organization of the pore structure in an anodic alumina template involves two coupled processes: pore formation with uniform diameters and pore ordering. The pores form with uniform diameters because of a delicate balance between electric-field-enhanced diffusion which determines the growth rate of the alumina, and dissolution of the alumina into the acidic electrolyte [8]. The pores are believed to self-order because of mechanical stress at the aluminum-alumina interface due to expansion during the anodization. This stress produces a repulsive force between the pores, causing them to arrange in a hexagonal lattice [2]. Depending on the anodization conditions, the pore diameter can be systematically varied from < 10nm up to 200nm with a pore density in the range of 109{1011 pores/cm2 [5, 7, 9 & 10]. It has been shown by many groups that the pore size distribution and the pore ordering of the anodic alumina templates can be significantly improved by a two-step anodization technique [3 & 11]., where the aluminum oxide layer is dissolved after the first anodization in an acidic solution followed by a second anodization under the same conditions.

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[11]. Lin, Y.-M., Sun, X., Cronin, S., Zhang, Z., Ying, J. Y., and Dresselhaus, M. S. (2000b). Fabrication and transport properties of Te-doped bismuth nanowire arrays. In Pantelides, S. T., Reed, M. A., Murday, J., and Aviran, A., editors, Molecular Electronics: MRS Symposium Proceedings, Boston, December 1999, volume 582, pages H10.3 (1-6), Pittsburgh, PA. Materials Research Society Press.

Other Templates

There are other porous materials that can be used as host templates for nanowire growth, as discussed by [1]. Nano-channel glass (NCG), for example, contains a regular hexagonal array of capillaries similar to the pore structure in anodic alumina with a packing density as high as 3x1010 pores/cm2 [2]. Porous Vycor glass that contains an interconnected network of pores less than 10nm was also employed for the early study of nanostructures [3]. Mesoporous molecular sieves [4]., termed MCM-41, possess hexagonally-packed pores with very small channel diameters which can be varied between 2nm and 10 nm. Conducting organic filaments have been fabricated in the nanochannels of MCM-41[5]. Recently, the DNA molecule has also been used as a template for growing nanometer-sized wires [6].

Diblock copolymers, which consist of two different polymer chains with different properties, have also been utilized as templates for nanowire growth. When two components are immiscible in each other, phase segregation occurs, and depending on their volume ratio, spheres, cylinders and lamellae may self-assemble. To form self-assembled arrays of nanopores, copolymers composed of polystyrene and polymethylmethacrylate [P(S-b-MMA)] were used [7]. By application of an electric field while the copolymer was heated above the glass transition temperature of the two constituent polymers, the self-assembled cylinders of PMMA could be aligned with their main axis perpendicular to the film. Selective removal of the PMMA component afforded the preparation of 14-nm-diameter ordered pore arrays with a packing density of 1.9 x 1011 cm-3.

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Nanowire Growth by Pressure Injection

The pressure injection technique is often employed for fabricating highly crystalline nanowires from a low-melting point material or when using porous templates with robust mechanical strength. In the high-pressure injection method, the nanowires are formed by pressure injecting the desired material in liquid form into the evacuated pores of the template. Due to the heating and the pressurization processes, the templates used for the pressure injection method must be chemically stable and be able to maintain their structural integrity at high temperatures and at high pressures. Anodic aluminum oxide films and nano-channel glass are two typical materials used as templates in conjunction with the pressure injection filling technique. Metal nanowires (Bi, In, Sn, and Al) and semiconductor nanowires (Se, Te, GaSb, and Bi2Te3) have been fabricated in anodic aluminum oxide templates using this method [1-3].

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[2]. Zhang, Z., Ying, J. Y., and Dresselhaus, M. S. (1998). Bismuth quantum-wire arrays fabricated by a vacuum melting and pressure injection process. J. Mater. Res., 13:1745-1748.

[3]. Lin, Y.-M., Cronin, S. B., Ying, J. Y., Dresselhaus, M. S., and Heremans, J. P. (2000). Transport properties of Bi nanowire arrays. Appl. Phys. Lett., 76:3944-3946.

Electrochemical Deposition

An attractive synthesis method, electrochemical deposition (ECD) is controllable and inexpensive, and provides great opportunities for the preparation of new materials and nanostructures [1-4].The electrochemical deposition technique has attracted increasing attention as a promising alternative for fabricating nanowires. Traditionally, electrochemistry has been used to grow thin films on conducting surfaces. Since electrochemical growth is usually controllable in the direction normal to the substrate surface, this method can be readily extended to fabricate 1D or 0D nanostructures, if the deposition is confined within the pores of an appropriate template. In the electrochemical methods, a thin conducting metal film is first coated on one side of the porous membrane to serve as the cathode for electroplating. The length of the deposited nanowires can be controlled by varying the duration of the electroplating process.

In the electrochemical deposition process, the chosen template has to be chemically stable in the electrolyte during the electrolysis process. Cracks and defects in the templates are detrimental to the nanowire growth, since the deposition processes primarily occur in the more accessible cracks, leaving most of the nanopores unfilled. Particle track-etched mica films or polymer membranes are typical templates used in the simple dc electrolysis. To use anodic aluminum oxide films in the dc electrochemical deposition, the insulating barrier layer which separates the pores from the bottom aluminum substrate has to be removed, and a metal film is then evaporated onto the back of the template membrane [5].

It is also possible to employ an ac electrodeposition method in anodic alumina templates without the removal of the barrier layer, by utilizing the rectifying properties of the oxide barrier. In ac electrochemical deposition, although the applied voltage is sinusoidal and symmetric, the current is greater during the cathodic half-cycles, making deposition dominant over the etching, which occurs in the subsequent anodic half-cycles. Since no rectification occurs at defect sites, the deposition and etching rates are equal, and no material is deposited. Hence, the difficulties associated with cracks are avoided.

In the case of materials prepared by the electrochemical method, besides the condition that can be easily used for the process development, the quality of the synthesized material can be better controlled by fine-tuning the electrolyte composition and electrolysis parameters control such as: the applied potential, the current density, electrical charge, temperature and the type of the electrolysis (potentiostatic or galvanostatic).

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Vapor Deposition

Vapor deposition of nanowires includes physical vapor deposition (PVD) [1], chemical vapor deposition (CVD) [2], and metallorganic chemical vapor deposition (MOCVD) [3].

An especially designed experimental setup given by Heremans et al. [1] has been used in the physical vapor deposition technique. The material to be filled is first heated to produce a vapor, which is then introduced through the pores of the template and cooled to solidify.

Compound materials that result from two reacting gases have also been prepared by the chemical vapor deposition (CVD) technique. In this method, the nanochannels are filled with one liquid precursor (e.g., Me3Ga or Et3In) via a capillary effect and the nanowires are formed within the template by reactions between the liquid precursor and the other gas reactant (e.g., AsH3).

[1]. Heremans, J., Thrush, C. M., Lin, Y.-M., Cronin, S., Zhang, Z., Dresselhaus, M. S., and Mansfield, J. F. (2000). Bismuth nanowire arrays: synthesis and galvanomagnetic properties. Phys. Rev. B, 61:2921-2930.

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[3]. Berry, A. D., Tonucci, R. J., and Fatemi, M. (1996). Fabrication of GaAs and InAs wires in nanochannel glass. Appl. Phys. Lett., 69:2846-2848.

Vapor-Liquid-Solid Method

Some of the recent successfully synthesized semiconductor nanowires are based on the so-called vapor-liquid-solid (VLS) mechanism of anisotropic crystal growth. This mechanism was first proposed for the growth of single crystal silicon whiskers 100 nm to hundreds of microns in diameter. The proposed growth mechanism involves the absorption of source material from the gas phase into a liquid droplet of catalyst (a molten particle of gold on a silicon substrate in the original work [1]. Upon super saturation of the liquid alloy, a nucleation event generates a solid precipitate of the source material. This seed serves as a preferred site for further deposition of material at the interface of the liquid droplet, promoting the elongation of the seed into a nanowire or a whisker, and suppressing further nucleation events on the same catalyst. Since the liquid droplet catalyzes the incorporation of material from the gas source to the growing crystal, the deposit grows anisotropically as a whisker whose diameter is dictated by the diameter of the liquid alloy droplet.

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Properties and Applications

Nanowires owing to their high density of electronic state, diameter-dependent band gap, enhanced surface scattering of electrons and phonons, increased excitation binding energy, high surface to volume ratio and large aspect ratio, nanowires of metals and semiconductor exhibit unique electrical, magnetic, optical, thermoelectric and chemical properties compared to their bulk parent counterparts. The interesting properties of nanowires hold lot of promises for applications in the fields of electronics, optics, magnetic medium, thermoelectronic, sensor devices etc. [1 & 2].

Magnetic Properties

The magnetic nanowires represent a class of nanosized materials in the shape of nanowires intensively studied in the last years is. This family of nanowires is interesting because of their magnetical and transport properties (giant magnetoresistance, reversal magnetization in only one nanowire) being of significant interest due to their potential to work as sensing elements in chemical biological sensors or in optical and electronic devices. The special properties of nanowires can be used in various applications (spintronics, miniaturization of magnetic sensors, ultrahigh-density magnetic storage media, etc.).

The interesting physical properties of magnetic nanowires reside in their geometry and in their dimensionality. The studies presented in the literature on simple magnetic nanowires based on Fe, Co and Ni show that the magnetic properties of nanowires materials are different from the bulk material. This is especially related to the shape anisotropy [3-5]. The giant magnetoresistance (GMR) studies of magnetic nanowire arrays started in the nineties [6] and is continuing nowadays [7 & 8].

Hexagonally arranged Ni-nanowires embedded in anodic alumina templates have been found to exhibit a strong enhancement in their magnetooptical (MO) response [9]. It has been reported [10] that the magnetic nanowires of Fe, Co, Ni show much enhanced magnetic coercivity than that of their bulk counterpart. It is important to note that the coercivity is strongly influenced by annealing of the wire at different temperatures, the aspect ratio and the wire diameter.

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Thermoelectric Properties

Nanowires are predicted to be promising for thermoelectric applications [1 & 2], due to their novel band structure compared to their bulk counterparts and the expected reduction in thermal conductivity associated with enhanced boundary scattering. Metal nanowires exhibit many fold increase in Seebeck coefficient due to their enhanced density of electronic states at the one-dimensional sub-band edges, which is attributed to the quantum confinement effect. It has been reported [3 & 4] that in Sb and Si doped Bi nanowires; the thermopower can be increased by decreasing the wire diameter. A similar phenomenon has also been observed in case of alumina doped Zn nanowires [3].

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Optical Properties

Optical methods provide an easy and sensitive tool for measuring the electronic structure of nanowires, since optical measurements require minimal sample preparation (for example, contacts are not required) and the measurements are sensitive to quantum effects. Optical spectra of 1D systems often show intense features at specific energies near singularities in the joint density of states which are formed under strong quantum confinement conditions. A variety of optical techniques have shown that the properties of nanowires are different from those of their bulk counterparts. A wide range of optical techniques are available for the characterization of nanowires, to distinguish their properties from those of their parent bulk materials. Some differences in properties relate to geometric differences, such as the small diameter size and the large length to diameter ratio (also called the aspect ratio), while others focus on quantum confinement issues.

The optical properties of nanowires have been studied extensively by employing different optical characterization and analytical techniques. The complex dielectric function (ε1 + ε2) of the nanowires which are embedded in the host material are deduced with the help of effective medium theories [1-4] by considering the nanowires and the host matrix to act as a single material. The refractive index (n) and the absorption coefficient (k) of the medium are related to ε1 and ε2, respectively for the composite medium. The complex dielectric function of the nanowires can also be determined directly with the help of standard reflection and transmission measurements combined with Maxwell's equations [5]. The band gap and temperature variation of band gap of the nanowires can be determined from the complex refractive index measurements [6], which are considered to be important parameters for selection of materials for particular photonic applications.

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Electrical Properties

Electron transport properties of nanowires are very important for electrical and electronic applications as well as for understanding the unique one-dimensional carrier transport mechanism. It has been noticed that the wire diameter, wire surface condition, crystal structure and its quality, chemical composition, crystallographic orientation along the wire axis etc. are important parameters, which influence the electron transport mechanism of nanowires.

It has been reported that quasi one-dimensional nanowires exhibit both ballistic and diffusive type electron transport mechanism, which depends upon the wire length and diameter. Ballistic type transport phenomena is associated with predominant carrier flow without scattering which is due to the fact that the carrier mean free path is longer than that of the wire length [1]. Ballistic type transport mechanism is normally observed at the contact junction of nanowire and other external circuits [2 & 3], where the conductance is quantized into an integral multiple of 2e2/h, called the universal conductance unit (e is the electronic charge and h the Plank's constant) [4 & 5].

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Applications

It is of great interest to find applications for nanowires, which could benefit in unprecedented ways from both the unique and tunable properties of nanowires and the small size of these nanostructures, for use in the miniaturization of conventional devices. As the synthetic methods for the production of nanowires are maturing and nanowires can be made in reproducible and cost-effective ways, it is only a matter of time before applications will be explored seriously. Commercialization of nanowire devices, however, will require reliable mass-production, effective assembly techniques and quality-control methods.

In this section, applications of nanowires to electronics, thermoelectrics, Optics, chemo- and bio-sensing, and magnetic media are discussed.

Electrical Applications

The self-assembly of nanowires might present a way to construct unconventional devices that do not rely on improvements in photo-lithography and, therefore, do not necessarily imply increasing fabrication costs. Devices made from nanowires have several advantages over those made by photolithography. A variety of approaches have been devised to organize nanowires via self-assembly. The unlike traditional silicon processing, different semiconductors can be simultaneously used in nanowire devices to produce diverse functionalities. Not only can wires of different materials be combined, but a single wire can be made of different materials. For example, junctions of GaAs and GaP show rectifying behavior [1], thus demonstrating that good electronic interfaces between two different semiconductors can be achieved in the synthesis of multicomponent nanowires.

Device functionalities common in conventional semiconductor technologies, such the p-n junction diodes [2], field-effect transistors [3], logic gates [2], and light-emitting diodes [1 & 4], have been recently demonstrated in nanowires, showing their promise as the building blocks toward the construction of complex integrated circuits by employing the \bottom-up" paradigm. Several approaches have been investigated to form nanowire diodes. For example, Schottky diodes can be formed by contacting a GaN nanowire with Al electrodes [5]. Furthermore, p-n junction diodes can be formed at the crossing of two nanowires, such as the crossing of n and p-type InP nanowires doped by Te and Zn, respectively [4], or Si nanowires doped by phosphorus (n-type) and boron (p-type) [6]. In addition to the crossing of two distinctive nanowires, heterogeneous junctions have also been constructed inside a single wire, either along the wire axis in the form of a nanowire superlattice [1], or perpendicular to the wire axis by forming a core-shell structure of silicon and germanium [7].

Nanowires have also been proposed for applications associated with electron field emission [8], such as flat panel display, because of their small diameter and large curvature at the nanowire tip, which may reduce the threshold voltage for the electron emission [9]. In this connection the demonstration of very high field emission currents from the sharp tip (»10nm radius) of a Si cone [8] and from carbon nanotubes [10] stimulates interest in this potential applications opportunity for nanowires.

[1]. Gudiksen, M. S., Lauhon, L. J., Wang, J., Smith, D., and Lieber, C. M. (2002). Growth of nanowire superlattice structures for nanoscale photonics and electronics. Nature, 415:617-620.

[2]. Huang, Y., Duan, X., Cui, Y., Lauhon, L. J., Kim, K.-H., and Lieber, C. M. (2001). Logic gates and computation from assembled nanowire building blocks. Science, 294:1313-1317.

[3]. Duan, X., Huang, Y., and Lieber, C. M. (2002). Nonvolatile memory and programmable logic from molecule-gated nanowires. Nano Lett., 2:487-490.

[4]. Duan, X., Huang, Y., Cui, Y., Wang, J., and Lieber, C. M. (2001). Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature, 409:66-69.

[5]. Kim, J.-R., Oh, H., So, H. M., Kim, J.-J., Kim, J., Lee, C. J., and Lyu, S. C. (2002). Schottky diodes based on a single GaN nanowire. Nanotechnology, 13:701-704.

[6]. Cui, Y. and Lieber, C. M. (2001). Functional nanoscale electronic devices assembled using silicon nanowire building blocks. Science, 291:851-853.

[7]. Lauhon, L. J., Gudiksen, M. S., Wang, D., and Lieber, C. M. (2002). Epitaxial core-shell and core-multishell nanowire heterostructures. Nature, 420:57-61.

[8]. Ding, M., Kim, H., and Akinwande, A. I. (1999). Observation of valence band electron emission from n-type silicon field emitter arrays. Appl. Phys. Lett., 75:823-825.

[9]. Au, F. C. K., Wong, K. W., Tang, Y. H., Zhang, Y. F., Bello, I., and Lee, S. T. (1999).

Electron field emission from silicon nanowires. Applied Physics Letters, 75:1700{1702.

[10]. Ajayan, P. M. and Zhou, O. Z. (2001). Applications of carbon nanotubes. In Dresselhaus, M. S., Dresselhaus, G., and Avouris, P., editors, Carbon Nanotubes: Synthesis, Structure, Properties and Applications, volume 80, pages 391{425, Berlin. Springer-Verlag. Springer Series in Topics in Applied Physics, Vol.80.

Thermoelectric Applications

The enhanced thermopower and manifold increase in the Seebeck coefficient of nanowires make them very attractive for thermoelectric cooling system and energy conversion devices [1 & 2]. The application of nanowires to thermoelectrics seems very promising; these materials are still in the research phase of the development cycle and quite far from being commercialized.

[1]. Dresselhaus G, Dresselhaus M S, Zhang Z, Sun X, Ying J and Chen G 1998 17th Int. conf. thermoelectrics: Proc. ICT'98, Nagoya (ed.) K Koumoto (Piscataway: Institute of Electrical and Electronics Engineers Inc) pp 43-46.

[2]. Chen G, Dresselhaus M S, Dresselhaus G, Fluerial J P and Caillat T 2003 Int. Mater. Rev. 48 45.

Optical Applications

Nanowires also hold promise for optical applications. One-dimensional systems exhibit a singularity in their joint density of states, allowing quantum effects in nanowires to be optically observable, sometimes, even at room temperature. Since the density of states of a nanowire in the quantum limit (small wire diameter) is highly localized in energy, the available states quickly fill up with electrons as the intensity of the incident light is increased. This filling up of the sub bands, as well as other effects that is unique to low-dimensional materials, lead to strong optical non-linearities in quantum wires. Quantum wires may thus yield optical switches with a lower switching energy and increased switching speed compared to currently available optical switches.

Uniform morphology and interesting optical properties of nanowires have raised their potential for various optical applications. The n-p junction of nanowires has been found to be capable of light emission, by virtue of their photoluminescence (PL) or electroluminescence (EL) properties. The use of p-n junction nanowires has been contemplated for laser applications. It has been established that ZnO nanowires of wire diameter smaller than the wavelength of emitted light exhibits lasing actions [1 & 2] at lower threshold energy compared to their bulk counterpart. This has been attributed to the exciton confinement effect in the laser action, which decreases the threshold lasing energy in nanowires. This effect has been observed in small diameter ZnO (385 nm diameter) and GaN nanowires [2 & 3]. The n-p junction nanowires or superlattice nanowires with p-n junctions can also be used as light emitting diodes [4 & 5]. The huge surface area and the high conductivity along the length of nanowires are suitable for inorganic-organic solar cell [6]. The solar cell made of CdSe nanowires has high efficiency [7].

Nanowires made of various metal segments like Ag,Au, Ni, Pd etc can be used as barcode tags [8] for different optical read outs.

[1]. Huang M H et al 2001 Science 292 1897

[2]. Johnson J C, Choi H J, Knutsen K P, Schaller R D, Yang P and Saykally R J 2002 Nature Mater. 1 106

[3]. Johnson J C, Yan H, Schaller R D, Haber L H, Saykally R J and Yang P 2001 J. Phys. Chem. B105 11387

[4]. Duan X, Huang Y, Cui Y, Wang J and Lieber C M 2001 Nature 409 66

[5]. Zhao J, Buia C, Han J and Lu J P 2003 Nanotechnology 14 501.

[6]. Huynh W U, Dittmer J J and Alivisatos A P 2002 Science 295 2425

[7]. Wu Y, Fan R and Yang P 2002 Nano. Lett. 2 83.

[8]. Nicewarner Pena S R et al 2004 Encyclopedia of nanoscience and nanotechnology (Valencia, California, USA: American Scientific Publishers) Vol. 6, p. 215

Chemical and Biochemical Sensing Devices

Sensors for chemical and biochemical substances with nanowires as the sensing probe are a very attractive application area. Nanowire sensors will potentially be smaller, more sensitive, demand less power, and react faster than their macroscopic counterparts. Arrays of nanowire sensors could in principle achieve nanometer scale spatial resolution and therefore provide accurate real-time information regarding not only the concentration of a specific analyte, but also its spatial distribution, as well as providing the corresponding information on other analytes within the same submicron volume. The development of nanowire based pH sensor [1] and Pb nanowire based hydrogen gas sensor [2] have been reported so far.

[1]. Cui Y, Wei Q, Park H and Lieber C 2001 Science 293 1289

[2]. Favier F, Walter E C, Zach M P, Benter T and Penner R M 2001 Science 293 2227

Magnetic Applications

It has been demonstrated that arrays of single domain magnetic nanowires can be prepared with controlled nanowire diameter and length, aligned along a common direction and arranged in a close-packed ordered array and that the magnetic properties (coercivity, remanence and dipolar magnetic interwire interaction) can be controlled to achieve a variety of magnetic applications [1 & 2].

The most attractive potential applications of nanowires lie in the magnetic information storage medium. Studies have shown that periodic arrays of magnetic nanowire arrays possess the capability of storing 1012 bits/in2 of information per square inch of area. The small diameter, single domain nanowires of Ni, Co fabricated into the pores of porous anodic alumina [1 & 2] has been found to be most suitable for the above purpose. The high aspect ratio of the nanowires results in enhanced coercivity and suppresses the onset of the "super paramagnetic limit", which is considered to be very important for preventing the loss of magnetically recorded information between the nanowires. Suitable separation between the nanowires is maintained to avoid the inter-wire interaction and magnetic dipolar coupling. It has been found that nanowires can be used to fabricate stable magnetic medium with packing density > 1011 wires/cm2.

[1]. Thurn-Albrecht, T., Schotter, J., KÄastle, G. A., Emley, N., Shibauchi, T., Krusin-Elbaum,L., Guarini, K., Black, C. T., Tuominen, M. T., and Russell, T. P. (2000). Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates. Science, 290:2126-2129.

[2]. Nielsch, K., Wehrspohn, R., Kronmuller, S. F. H., Barthel, J., Kirschner, J., and Gosele, U. (2001). Magnetic properties of 100nm nickel nanowire arrays obtained from ordered porous alumina templates. MRS Symp. Proc., 636:D1.9 (1-6).

Characterization of Nanowires

The discovery and investigation of nanostructures were spurred on by advances in various characterization and microscopy techniques that enable materials characterization to take place at smaller and smaller length scales, reaching length scales down to individual atoms. For applications, characterization of the nanowire structural properties is especially important so that a reproducible relationship between their desired functionality and their geometrical and structural characteristics can be established. Due to the enhanced surface to volume ratio in nanowires, their properties may depend sensitively on their surface condition and geometrical configuration. Even nanowires made of the same material may possess dissimilar properties due to differences in their crystal phase, crystalline size, surface conditions, and aspect ratios, which depend on the synthesis methods and conditions used in their preparation.

X-Ray Diffraction

X-Ray Diffraction (XRD) the characterization technique is commonly used to study the crystal structure and chemical composition of nanowires. The peak positions in the x-ray diffraction pattern can be used to determine the chemical composition and the crystal phase structure of the nanowires.

X-rays are electromagnetic radiation similar to light, but with a much shorter wavelength. They are produced when electrically charged particles of sufficient energy are decelerated. In an X-ray tube, the high voltage maintained across the electrodes draws electrons toward a metal target (the anode). X-rays are produced at the point of impact, and radiate in all directions. Tubes with copper targets, which produce their strongest characteristic radiation (Khttp://pubs.usgs.gov/of/2001/of01-041/htmldocs/icons/alpha.gif1) at a wavelength of about 1.5 angstroms, are commonly used for geological applications.

Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law

This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By changing the geometry of the incident rays, the orientation of the centered crystal and the detector, all possible diffraction directions of the lattice should be attained.

D:\Users\Babar\Desktop\xrdtube.jpg

Fig. Schematic Cross section of an x-ray tube [1]

[1]. http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/xrdtube.jpg

Scanning Electron Microscopy

SEM usually produces images down to length scales of >10nm and provides valuable information regarding the structural arrangement, spatial distribution, wire density, and geometrical features of the nanowires.

The concept of a Scanning Electron Microscope was first described by Knoll in 1935. Basically a SEM is built-up of a column on a sample chamber. At the top of the column, electrons are generated. These electrons are focused on the sample by means of condensers and coils. The electron beam is scanned over the sample. Around the sample, detectors sense the different signals generated by the electron beam. The signals detected are used to generate an image on the computer screen. What basically happens is that the electron beam scans an array of pixels. Every pixel is filled with a grey-value from one of the detectors. The most important signals that are detected are Secondary Electrons (SE) (relatively slow electrons that lost part of their energy in the sample), Backscattered Electrons (BSE) (electrons that retained most of their energy) and X-rays. SE gives topographic contrast, which is topographic information about the surface of the sample. BSE gives compositional contrast, which is information about the composition of the sample: heavier elements appear brighter on the screen.

Operation

In SEM, a source of electrons is focused in vacuum into a fine probe that is rastered over the surface of the specimen. The electron beam passes through scan coils and objective lens that deflect horizontally and vertically so that the beam scans the surface of the sample (shown in Figure). As the electrons penetrate the surface, a number of interactions occur that can result in the emission of electrons or photons from or through the surface. A reasonable fraction of the electrons emitted can be collected by appropriate detectors, and the output can be used to modulate the brightness of a cathode ray tube (CRT) whose x- and y- inputs are driven in synchronism with the x-y voltages rastering the electron beam. In this way an image is produced on the CRT; every point that the beam strikes on the sample is mapped directly onto a corresponding point on the screen [1]. As a result, the magnification system is simple and linear magnification is calculated by the equation:

where L is the raster's length of the CRT monitor and l the raster's length on the surface of the sample. SEM works on a voltage between 2 to 50kV and its beam diameter that scans the specimen is 5nm-2μm. The principle images produced in SEM are of three types: secondary electron images, backscattered electron images and elemental X-ray maps. Secondary and backscattered electrons are conventionally separated according to their energies. When the energy of the emitted electron is less than about 50eV, it is referred as a secondary electron and backscattered electrons are considered to be the electrons that exit the specimen with an energy greater than 50eV [2]. Detectors of each type of electrons are placed in the microscope in proper positions to collect them.

D:\Users\Babar\Desktop\sem2.gif

Fig. Geometery of SEM [3]

[1]. R.F. Egerton. Electron Energy-Loss Spectroscopy in the Electron Microscope.

[2]. C. Richard Brundle, Charles A. Evans Jr, Shaun Wilson. Encyclopedia of materials characterization, Butterworth-Heinemann publications, 1992.

[3]. http://www.purdue.edu/rem/rs/sem.htm

Literature Review

Enormous interest in the synthesis of nanowires has been grown in the last few decades. Different experimental techniques have been used to grow the nanowires for many materials. Silver (Ag) nanowires grown by DNA-template, redox template, pulsed Electrochemical Deposition (ECD) [1 & 2], Gold (Au) by template EDC [3 & 4], Bismuth (Bi) was synthesized by stress-induced [5], template vapor-phase [6], ECD [7-9] and pressure-injection [10-12].

Many studies have focused on the fabrication of Copper (Cu) nanowires [13-17], because of their potential applications in the micro/nanoelectronics industry and, in particular, for interconnection in electronic circuits. Cu nanowires fabricated by vapor deposition [18], and ECD [19]. Silicon nanowires were prepared by Vapor-liquid-solid (VLS) [20], laser-ablation VLS [21], oxide-assisted [22] and low-T VLS [23]. Templates were used to deposit zinc nanowires by vapor-phase [24] and EDC [25] methods. Nickel and iron nanowires were electrochemically deposited by using the template [26-30]. Many alloys nanowires of different materials were grown by many techniques: GaN by Chemical vapor deposition (CVD) [31] and VLS [32 & 33], GaAs by liquid/vapor Organometallic chemical vapor deposition (OMCVD) [34], InAs by liquid/vapor OMCVD [34], InP by VLS [35], PbSe by liquid phase [36], ZnO by VLS [37] and [38 & 39], Bi2Te3 by dc EDC [40], CdS by liquid-phase (surfactant), recrystallization [41] and ac EDC [42 & 43] and CdSe by liquid-phase (surfactant), redox [44], and ac EDC [45 & 46].

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[28]. Yin, A. J., Li, J., Jian, W., Bennett, A. J., and Xu, J. M. (2001). Fabrication of highly ordered metallic nanowire arrays by electrodeposition. Appl. Phys. Lett., 79:1039-1041.

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